1. Field of the Invention
The present invention relates to a soft intraocular lens material. More specifically, it relates to a soft intraocular lens material which is excellent in flexibility and has a high refractive index, whereby the lens can be made thin and can be folded so that it can be inserted through a small incision, and which is yet excellent in transparency and free from so-called glistenings i.e. a phenomenon whereby such transparency is lost.
2. Discussion of Background
In order to minimize the injury to the eye by cataract operation using an intraocular lens, it is usually advisable to minimize the incision for the surgical operation. As phacoemulsification has progressed wherein a crystalline lens is fractured by ultrasonic vibration, and its fragments are suctioned by means of a small cannula, it has been made possible to remove a crystalline lens through an incision not larger than 2 or 3 mm.
However, an intraocular lens usually has a diameter of about 6 mm, and to insert such an intraocular lens as it is, it is necessary to incise the portion for insertion to a large extent. Accordingly, in recent years, various intraocular lenses of soft type which are soft, flexible and swellable, have been invented, whereby insertion through a small incision has been made possible.
As such intraocular lenses, there have been proposed, for example, an intraocular lens obtained by copolymerizing a monomer mixture comprising at least two types of (meth)acrylate monomers having aromatic rings, and a crosslinkable monomer (JP-A-4-292609), a soft intraocular lens obtained by using components comprising a perfluorooctylethyloxypropylene (meth)acrylate monomer, a 2-phenylethyl (meth)acrylate monomer, an alkyl (meth)acrylate and a crosslinkable monomer (JP-A-8-224295), and an intraocular lens obtained by using a monomer, of which the refractive index of the homopolymer is at least 1.5, a monomer, of which the glass transition temperature of the homopolymer is lower than 30.degree. C., and a crosslinkable monomer (JP-A-8-503506).
Each of these intraocular lenses has transparency and flexibility and is deformable, and each of them can accordingly be inserted through a relatively small incision. However, they have a drawback that when they are implanted in the eyes, white dots appear in the lenses by hydration, whereby so-called glistenings take place i.e. a phenomenon whereby the transparency is substantially lowered or lost.
In addition to those mentioned above, for example, an intraocular lens material employing a hydroxyalkyl (meth)acrylate and a ring structure- or halogen atom-containing (meth)acrylate and/or (meth)acrylamide derivative as the main components (JP-A-6-22565), an ocular lens material for e.g. an intraocular lens, obtained by using a composition comprising a (meth)acrylate having a hydroxyl group and a phenoxy group which may be substituted, and a crosslinking agent (JP-A-8-173522), and a soft intraocular lens material employing an aromatic ring-containing acrylate and a fluorine atom-containing alkyl acrylate as the main components (JP-A-9-73052), have been proposed.
Each of these materials is excellent in flexibility and has a large shape-regaining force and thus can be inserted through a small incision. However, when they are implanted in the eyes, their transparency likewise tends to decrease by a temperature change. Therefore, it has been desired to develop an intraocular lens which is free from such a decrease in the transparency (glistenings).
Further, JP-A-7-24052 discloses a colored soft intraocular lens material obtained by adding a yellow colorant, as a soft intraocular lens for cyanopsia. Usually, a soft material can be subjected to extraction treatment with an organic solvent for the purpose of removing a non-polymerized component remaining in the interior of the material. However, the dispersion type material disclosed in JP-A-7-24052 has a problem that it undergoes elution during the extraction treatment, whereby the lens undergoes a color change.